Triglyceride oils contain undesirable minor components or impurities including free saturated fatty acids, such as palmitic or stearic acids, and other suspended matter that, unless removed, render the oil commercially unsuitable in that they produce a soapy taste or a strong flavour. Such unrefined oils are generally refined by one or several of the following steps: degumming, neutralizing or alkali refining to reduce the fatty acid content thereof, bleaching, dewaxing and deodorization.
The main source of haziness and discoloration in triglyceride oil is the presence of crystallized triglycerides with saturated fatty acids such as palmitic or stearic acids. These triglycerides with saturated fatty acids crystallize and agglomerate producing a haze and also precipitate creating a turbid product. Another cause of haziness in the oil is the presence of other dispersed solids like protein and mucilaginous materials of microscopic size. Precipitated matter, such as proteins, can cause deterioration of the oil. When these microscopic materials agglomerate they become visible and produce unsightly haze in the final oil product. The haziness due to crystallized saturated fat is not very aesthetically pleasing. This is detrimental, particularly in cosmetics and pharmaceuticals, since it is important for the oil to be very clear and translucent to be appealing to customers. In the field of oil processing, fractionation almost always refers to the mechanical separation of the liquid from the solid, crystallized, constituents of a given oil. The split between liquid and solid fractions depends on the temperature at which crystallization is conducted.
Fractionation is a process that has been known in industrial from for more than a century. Before then, the olein and stearin fractions had been separated by settling, using only the force of gravity to bring about a separation between the heavier solid phase and the lighter liquid phase. Naturally this method of fractionation left the settled solid phase containing large quantities of entrained or trapped liquid oil, almost certainly more than 15%.
In the latter years a process of this type, using only indirect cooling of the oil but separating liquid from solid by filter or centrifuge, become known as “dry fractionation”.
Vegetable oils especially palm oil is fractionated in one- or two-stages by utilizing the different in melting points of respective components, there have been known for instances, solvent fractionation using organic solvent such as acetone, hexane, or the like, detergent fractionation using a surfactant, dry fractionation, sweating and the like.
Among these, solvent fractionation is advantageous because fractionation can be carried out precisely. However, on the other hand, this is dangerous, since a flammable solvent is used, and also requires high production costs. In addition, solvent fractionation is not the most effective process for the fractionation of raw materials such as coconut oil, palm kernel oil and fat.
The method of detergent fractionation has inferior precision of fractionation and its products have inferior quality in comparison with those fractionated using solvent fractionations. Furthermore, separation of oil from an aqueous solution containing a surfactant and treatment of waste water containing a surfactant are troublesome and incomplete.
The method of dry fractionation requires expensive crystallization tank facilities. In addition, productivity, fractionation efficiency and quality of a product are inferior to those of the above two methods.
Application of the sweating method is limited to certain kinds of fats and oils. That is, it is employed for removing a wax but is not suitable for fractionation of oils or fats.
Dry fractionation involves the heating up of palm oil to a temperature of between 50 to 55° C., cooling the oil to between 30 to 40° C. followed by further cooling of the oil to the final fractionation temperature of between 20 to 25° C. The crystallizer is then held at this temperature for a number of hours depending on the type and characteristics of the olein and stearin desired. The crystallized slurry is then filtered under a pressure to obtain the olein and stearin fractions. The yield of olein and stearin obtained is between 75 to 80% and 20 to 25%, respectively. If the holding times, the number of fractionation steps or the filtration pressure is varied the characteristics of the olein and stearin obtained could be altered. The iodine value (IV) of the olein obtained is about 56 for a single fractionation of around 10 hours holding time at the final fractionation temperature and a filtration pressure of 3 to 5 bars.
Dry fractionation of refined palm oil using the conditions stated above is deemed to be difficult to control due to the presence of gums and other impurities which will interfere with the crystallization of the oil during the fractionation process.
At present the fractionation of refined palm oil is only carried out using the wet detergent process. An aqueous solution of sodium lauryl sulphate is added and the mixture is cooled to crystallize the stearin. The slurry is then centrifuged to separate the solid from the liquid phase. Water is then removed from the olein phase and the detergent removed at the same time. This process is completely different from that of dry fractionation. It may be very difficult to completely remove all the detergent from the olein phase and there may be trace quantities of the detergent left. In view of the mounting emphasis on food safety in the future, this process will be less and less appealing.
U.S. Pat. No. 4,795,569 to Higuchi et al. describes a process in which the oil is introduced into a filter chamber and allowed to crystallize inside that chamber by circulating a coolant such as water through the space between the membrane and a filter frame. However, this process requires the filter cloth to be sealed first with a coagula of the material to be treated. This makes it a lengthy process that makes inefficient use of the expensive membrane press.
Accordingly, an improvement has been described in U.S. Pat. No. 5,045,243 to Kuwabara et al. in which the oil or fat to be fractionated is first of all solidified in trays to form solid blocks which are then crushed to yield a pumpable paste that is then introduced into a membrane press to separate this paste into an olein fraction and a stearin fraction. The solidification process is commonly carried out in cooling tunnels. However, these have the disadvantage that the oil is exposed to the air while being and that it is virtually impossible to control the rate of cooling inside the individual trays.
Therefore, European Patent Application 1028159 by Yoneda et al. disclosed a stationary crystallisation. The oil or fat to be fractionated is not solidified into a solid block, but the crystallisation process is halted when the partially crystallised mass is still sufficiently fluid to be pumped into the membrane filter press. However, this means that the material to be fractionated has to be diluted with olein before being cooled.